Sale post Registered Will

We purchased a property via Sale Deed in 2014 at Delhi. The Property structure is G+2. The owner wrote a registered will in favour of his 3 sons where Son 1 gets Gr Floor, Son 2 gets 1st flr and Son 3 gets 2nd Flr. It was also mentioned in the will that before any son sells his property, 1st he will offer the property as per market rates to his brothers and if they are not interested to purchase he may go ahead and sell to an outsider. Son 2 sold his complete first floor rights along with 33.3% land share to us and also mentioned in the sale deed that he first approached both his brothers and since they are not interested to purchase, he then is selling to us. However he has not taken any physical NOC from them. It has been more than 8 years now since we are staying in this property post registry and Son 1 and Son 3 still lives at their respective floors. So now my question is that in future can any of the son still object or challenge the sale deed stating that they have not provided NOC or the seller did not ask him to purchase the property before selling to us ?